TEL AVIV (Reuters) - U.S. Defense Secretary Jim Mattis said on Friday that Syria had dispersed its warplanes in recent days and that it retained chemical weapons, an issue he said would have to be taken up diplomatically.

The United States launched dozens of missiles earlier this month against a Syrian air base in response to a chemical attack that killed 90 people, including 30 children. It says the Syrian government launched the attack from the Shayrat air base.

The Pentagon has said that the strike had damaged or destroyed about 20 percent of the Syrian military's operational aircraft.

During a press conference alongside his Israeli counterpart, Mattis was asked whether the Syrian military had moved warplanes to a Russian base in Latakia.

"They have dispersed their aircraft, no doubt. They have dispersed their aircraft in recent days," Mattis said.

Mattis also reiterated that the United States believed Syria had retained some chemical weapons.

"The bottom line is, I can say authoritatively they have retained some (chemical weapons). It's a violation of the United Nations Security Council resolutions, and it's going to have to be taken up diplomatically," Mattis said.

Israel's military said on Wednesday it believes Syrian President Bashar al-Assad's forces still possess several tonnes of chemical weapons.

A senior Israeli military officer told Israeli reporters that "a few tonnes of chemical weapons" remained in the hands of Assad's forces, a military official told Reuters.

In a 2013 agreement brokered by Russia and the United States, Syria agreed to destroy its chemical weapons.

The Organisation for the Prohibition of Chemical weapons, a global watchdog, said sarin or a similar banned toxin was used in the April 4 strike in Syria's Idlib province.

Mattis later met Israeli Prime Minister Benjamin Netanyahu in Jerusalem. Before the start of their talks, Netanyahu said he was optimistic about relations between the two countries under the new U.S. administration.

The two countries are working to set a more positive tone after eight years of friction under President Donald Trump's Democratic predecessor, Barack Obama.